Late blight, caused by the oomycete Phytophthora infestans is one of the most serious diseases in worldwide potato production. It was responsible for the Irish potato famine of the mid-19th century, resulting in the death of one million people. Although a lot of effort has been invested in controlling the pathogen, chemical control of P. infestans is still the main crop management strategy, but environmental safety is becoming more important and the pathogen is sometimes able to evolve chemical resistance. Therefore, introduction of resistance into modern potato varieties is the most durable strategy to control the disease.
In the last century, Solanum demissum, which is a hexaploid Mexican species, was extensively used in breeding for late-blight resistance in potato. Initially, a series of 11 R genes derived from S. demissum was described. Of these, R1, R2, R3a/b, R6, and R7 have been localized on the genetic maps of potato. However, these R genes confer race-specific resistance and those that were introgressed into potato varieties, mainly R1, R2, R3, R4, and R10, were quickly overcome by the pathogen. Hence, new sources for resistance are required, and currently, several other wild Solanum species have been reported as being potential sources of resistance, many of which have currently been genetically characterized (Table 1).
S. bulbocastanum, a self-incompatible diploid species from Mexico, is thought to be a source for late-blight resistance. Introduction of S. bulbocastanum derived resistance has been achieved through interspecific bridge crosses between S. bulbocastanum, S. acaule, S. phureja, and S. tuberosum (Hermsen and Ramanna, Euphytica 22: 457-466, 1973), resulting in so-called ABPT material that is widely used for potato late-blight breeding. Additionally, Helgeson et al (Theor. Appl. Genet. 96:738-742, 1998) generated somatic hybrids between S. bulbocastanum and cultivated potato. The somatic hybrids led to fertile plants that retained resistance and could be used for breeding. Molecular cloning of the genes responsible for resistance and subsequent introduction of the genes into potato varieties is a third method that circumvents many of the problems encountered in the previous two strategies.
To date, two R genes from S. bulbocastanum have been cloned, the allelic genes RB and Rpi-blb1 on chromosome 8 (Song et al. Proc. Natl. Acad. Sci 100: 9128-9133, 2003; van der Vossen et al. Plant Journal 36: 867-882, 2003) and Rpi-blb2 on chromosome 6 (van der Vossen et al. Plant Journal 44: 208-222, 2005). As shown in Table 5, Rpi-blb1 as well as Rpi-blb2 provide protection against a diverse set of Phytophthora infestans isolates. Until the present invention no Phytophthora isolates were described that could colonize plants harboring Rpi-blb1, hence the specification ‘broad spectrum resistance gene’ was used to describe the protection conferred by this gene. However, as disclosed in Table 5, Phytophthora isolate 99189 is able to grow on Rpi-blb1 plants and thus to ‘break’ the resistance, suggesting race specificity.